1. Field of the Invention
The invention relates to a device for producing a single crystal by crystallizing the single crystal in a melt zone. The device is suitable in particular for the production of a silicon single crystal.
2. Description of the Related Art
DE 30 07 377 A1 describes a known embodiment of a device for crystallizing a single crystal in a wet zone, and a method which is referred to as a zone melting method or FZ method. This embodiment comprises a housing in which a pulling shaft and an inductor are accommodated. At the upper end of the pulling shaft, there is a seed crystal on which molten silicon crystallizes, initially to form a section with a narrowed diameter and subsequently to form a single crystal. The inductor, an RF induction heating coil formed as a flat coil, causes polycrystalline silicon at the lower end of a rod to melt and generates and stabilizes a melt zone, which delivers the material that the single crystal requires in order to grow. The polycrystalline rod and the single crystal are lowered, in order to permit continuous growth of the single crystal.
A variant of the FZ (floating zone) method is the GFZ (granular floating zone) method, which is described for example in US 2011/0095018 A1. In the GFZ method, polycrystalline granules are used instead of a rod as raw material. Furthermore, two induction heating coils are provided, a first for melting the granules and a second for stabilizing the melt zone by generating heat in the melt zone.
The temperature field in the melt zone and in the cooling single crystal, particularly in the region of the crystallization boundary, must be controlled as precisely as possible. In particular, it is necessary to counteract steep temperature gradients which cause thermal stresses and may be the origin of dislocation formation even to the extent of destroying the single crystal by cracking. In order to control the temperature field, it has been found to be advantageous to provide a reflector which surrounds the growing single crystal and reflects thermal radiation onto it.
In addition to the use of a reflector, CN 10232191 A proposes to use a transverse magnetic field in order to suppress fluctuations of the temperature in the melt zone.